Partitioning Data Within A Distributed Data Storage System Using Virtual File Links

ABSTRACT

A record within a destination virtual file is generated on a destination node of a distributed data storage system. The record comprises (i) a link directed to a partition of a source virtual file stored on a source node and (ii) partition criteria characterizing the partition. The source virtual file is mapped to a chain of linked pages stored in a page buffer of the distributed data storage system and the partitioning criteria is used by at least one of the source node and the destination node to identify data associated with the partition. A request is later received at the destination node to access data defined by the destination virtual file. Data is provided, in response to the request, from the partition of the source virtual file stored on the source node using the link and the partitioning criteria. Related apparatus, systems, techniques and articles are also described.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/290,835 filed on Nov. 7, 2011, the contents of which arehereby fully incorporated by reference.

TECHNICAL FIELD

The subject matter described herein relates to techniques forpartitioning data and enabling access to partitioned data within adistributed data storage system using virtual file links.

BACKGROUND

In a distributed data storage system, data containers sometimes becomeso big that they must be partitioned/split over several nodes due tomemory limitations and/or performance issues. Partitioning a datacontainer means splitting a data container into several parts as well ascombining smaller data containers into a single data container andmoving associated data to another node. With some conventional databasesystems, this requires not only moving the data but also moving one ormore logs tracking changes to such data container. These operations canbe processor intensive and can also consume additional storage.

SUMMARY

In one aspect, a record is generated within a destination virtual fileon a destination node of a distributed data storage system. The recordcomprises (i) a link directed to a source virtual file stored on asource node and (ii) partition criteria characterizing a partition ofthe source virtual file. The source virtual file is mapped to a chain oflinked pages stored in a page buffer of the distributed data storagesystem. The partitioning criteria is used by at least one of the sourcenode and the destination node to identify data associated with thepartition. Thereafter, a request is received at the destination node toaccess data defined by the destination virtual file. In response to therequest, data is provided from partition of the source virtual filestored on the source node using the link and the partitioning criteria.

Data generated at the node subsequent to the generation of the recordcan be appended to the record. The appended data can form a delta log atthe destination node. The destination virtual file and the delta log canbe overwritten during a columnar table merge operation with a newversion of the destination virtual file comprising data from thepartition of the source virtual file and the delta log. The link in therecord can be overwritten during the columnar table merge operation. Thenew version of the destination virtual file can be persisted tosecondary data storage. The new version of the destination virtual filecan be persisted after a savepoint on the destination node. Thepartition of the source virtual file can be dropper after the newversion of the destination virtual file is persisted to the secondarydata storage. Dropping the partition of the source virtual file caninclude ceasing log operations relative to a portion of the sourcevirtual file corresponding to the partition and/or deleting a portion ofthe source virtual file corresponding to the partition.

Articles of manufacture are also described that comprise computerexecutable instructions permanently stored on non-transitory computerreadable media, which, when executed by a computer, causes the computerto perform operations herein. Similarly, computer systems are alsodescribed that may include a processor and a memory coupled to theprocessor. The memory may temporarily or permanently store one or moreprograms that cause the processor to perform one or more of theoperations described herein. In addition, operations specified bymethods can be implemented by one or more data processors either withina single computing system or distributed among two or more computingsystems.

The subject matter described herein provides many advantages. Forexample, the current techniques allow for the partitioning of data amongnodes within a distributed data storage system (i.e., a database systemcomprising a plurality of nodes, etc.) with little performance impact.Using links to virtual files as described herein obviates the need,during recovery from a log backup, for moving a portion of a virtualfile from one node to the other which in turn requires writing a redolog on a destination node for all moved data or explicit expensivesynchronization of recovery on several nodes. Stated differently, theuse of a link to a virtual file requires a single operation (reading thelinked virtual file) as opposed to physically moving data which requiresat least three operations (writing data from the virtual file to thedestination node, writing a log from the virtual file to the destinationnode, and reading data written to the destination node).

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a system including a data storageapplication;

FIG. 2 is a process flow diagram illustrating partioning data within adistributed data storage system using virtual file links;

FIG. 3 is a diagram illustrating details of the system of FIG. 1; and

FIG. 4 is a diagram illustrating partitioning of data on a source nodeto a destination node within a distributed data storage system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows an example of a system 100 in which a computing system 102,which can include one or more programmable processors that can becollocated, linked over one or more networks, etc., executes one or moremodules, software components, or the like of a data storage application104. The data storage application 104 can include one or more of adatabase, an enterprise resource program, a distributed storage system(e.g. NetApp Filer available from NetApp of Sunnyvale, Calif.), or thelike.

The one or more modules, software components, or the like can beaccessible to local users of the computing system 102 as well as toremote users accessing the computing system 102 from one or more clientmachines 106 over a network connection 110. One or more user interfacescreens produced by the one or more first modules can be displayed to auser, either via a local display or via a display associated with one ofthe client machines 106. Data units of the data storage application 104can be transiently stored in a persistence layer 112 (e.g. a page bufferor other type of temporary persistency layer), which can write the data,in the form of storage pages, to one or more storages 114, for examplevia an input/output component 116. The one or more storages 114 caninclude one or more physical storage media or devices (e.g. hard diskdrives, persistent flash memory, random access memory, optical media,magnetic media, and the like) configured for writing data for longerterm storage. It should be noted that the storage 114 and theinput/output component 116 can be included in the computing system 102despite their being shown as external to the computing system 102 inFIG. 1.

Data retained at the longer term storage 114 can be organized in pages,each of which has allocated to it a defined amount of storage space. Insome implementations, the amount of storage space allocated to each pagecan be constant and fixed. However, other implementations in which theamount of storage space allocated to each page can vary are also withinthe scope of the current subject matter.

FIG. 2 is a process flow diagram 200 in which, at 210, a record isgenerated on a destination node of a distributed data storage system.The record can include (i) a link directed to a source virtual filestored on a source node and (ii) partition criteria characterizing apartition of the source virtual file. The source virtual file can bemapped to a chain of linked pages stored in a page buffer of thedistributed data storage system, the partitioning criteria being used byat least one of the source node and the destination node to identifydata associated with the partition. Subsequently, at 220, a request isreceived at the destination node to access data defined by thedestination virtual file. In response to the request, at 230, data isprovided from the partition of the source virtual file stored on thesource node using the link and the partitioning criteria.

FIG. 3 shows a software architecture 300 consistent with one or morefeatures of the current subject matter. A data storage application 104,which can be implemented in one or more of hardware and software, caninclude one or more of a database application, a network-attachedstorage system, or the like. According to at least some implementationsof the current subject matter, such a data storage application 104 caninclude or otherwise interface with a persistence layer 112 or othertype of memory buffer, for example via a persistence interface 302. Apage buffer 304 within the persistence layer 112 can store one or morelogical pages 306, and optionally can include shadow pages 311, activepages 313, data pages of virtual files 315 and the like. The logicalpages 306 retained in the persistence layer 112 can be written to astorage (e.g. a longer term storage, etc.) 114 via an input/outputcomponent 116, which can be a software module, a sub-system implementedin one or more of software and hardware, or the like. The storage 114can include one or more data volumes 310 where stored pages 312 areallocated at physical memory blocks.

In some implementations, the data storage application 104 can include arow store 303 and a column store 305. The row store 303 can comprise orbe otherwise in communication with a page manager 314 and/or a savepointmanager 316. The page manager 314 can communicate with a page managementmodule 320 at the persistence layer 112 that can include a free blockmanager 322 that monitors page status information 324, for example thestatus of physical pages within the storage 114 and logical pages in thepersistence layer 112 (and optionally in the page buffer 304). Thesavepoint manager 316 can communicate with a savepoint coordinator 326at the persistence layer 204 to handle savepoints, which are used tocreate a consistent persistent state of the database for restart after apossible crash. The row store 303 can access the persistence interface302 via an absolute page API 307. The column store 305 which can storecolumns in contiguous memory can access the persistence interface 302via a virtual file API 309.

In some implementations of a data storage application 104, the pagemanagement module of the persistence layer 112 can implement shadowpaging. The free block manager 322 within the page management module 320can maintain the status of physical pages. The page buffer 304 canincluded a fixed page status buffer that operates as discussed herein. Aconverter component 340, which can be part of or in communication withthe page management module 320, can be responsible for mapping betweenlogical and physical pages written to the storage 114. The converter 340can maintain the current mapping of logical pages to the correspondingphysical pages in a converter table 342. The converter 340 can maintaina current mapping of logical pages 306 to the corresponding physicalpages in one or more converter tables 342. When a logical page 306 isread from storage 114, the storage page to be loaded can be looked upfrom the one or more converter tables 342 using the converter 340. Whena logical page is written to storage 114 the first time after asavepoint, a new free physical page is assigned to the logical page. Thefree block manager 322 marks the new physical page as “used” and the newmapping is stored in the one or more converter tables 342.

The persistence layer 112 can ensure that changes made in the datastorage application 104 are durable and that the data storageapplication 104 can be restored to a most recent committed state after arestart. Writing data to the storage 114 need not be synchronized withthe end of the writing transaction. As such, uncommitted changes can bewritten to disk and committed changes may not yet be written to diskwhen a writing transaction is finished. After a system crash, changesmade by transactions that were not finished can be rolled back. Changesoccurring by already committed transactions should not be lost in thisprocess. A logger component 344 can also be included to store thechanges made to the data of the data storage application in a linearlog. The logger component 344 can be used during recovery to replayoperations since a last savepoint to ensure that all operations areapplied to the data and that transactions with a logged “commit” recordare committed before rolling back still-open transactions at the end ofa recovery process.

With some data storage applications, writing data to a disk is notnecessarily synchronized with the end of the writing transaction.Situations can occur in which uncommitted changes are written to diskand while, at the same time, committed changes are not yet written todisk when the writing transaction is finished. After a system crash,changes made by transactions that were not finished must be rolled backand changes by committed transaction must not be lost.

To ensure that committed changes are not lost, redo log information canbe written by the logger component 344 whenever a change is made. Thisinformation can be written to disk at latest when the transaction ends.The log entries can be persisted in separate log volumes 317 whilenormal data is written to data volumes 310. With a redo log, committedchanges can be restored even if the corresponding data pages were notwritten to disk. For undoing uncommitted changes, the persistence layer112 can use a combination of undo log entries (from one or more logs)and shadow paging.

The persistence interface 302 can handle read and write requests ofstores (e.g., in-memory stores, etc.). The persistence interface 302 canalso provide write methods for writing data both with logging andwithout logging. If the logged write operations are used, thepersistence interface 302 invokes the logger 344. In addition, thelogger 344 provides an interface that allows stores (e.g., in-memorystores, etc.) to directly add log entries into a log queue. The loggerinterface also provides methods to request that log entries in thein-memory log queue are flushed to disk.

Log entries contain a log sequence number, the type of the log entry andthe identifier of the transaction. Depending on the operation typeadditional information is logged by the logger 344. For an entry of type“update”, for example, this would be the identification of the affectedrecord and the after image of the modified data.

When the data application 104 is restarted, the log entries need to beprocessed. To speed up this process the redo log is not always processedfrom the beginning. Instead, as stated above, savepoints can beperiodically performed that write all changes to disk that were made(e.g., in memory, etc.) since the last savepoint. When starting up thesystem, only the logs created after the last savepoint need to beprocessed. After the next backup operation the old log entries beforethe savepoint position can be removed.

When the logger 344 is invoked for writing log entries, it does notimmediately write to disk. Instead it can put the log entries into a logqueue in memory. The entries in the log queue can be written to disk atthe latest when the corresponding transaction is finished (committed oraborted). To guarantee that the committed changes are not lost, thecommit operation is not successfully finished before the correspondinglog entries are flushed to disk. Writing log queue entries to disk canalso be triggered by other events, for example when log queue pages arefull or when a savepoint is performed.

The column store 305 can persist its tables to virtual files provided bythe persistence layer 112 via the virtual file API 307. Internally thepersistence layer 112 can map a virtual file to a chain of linked pages315 stored in the page buffer 304. Data belonging to one columnar tablecan be stored in multiple virtual files: one virtual file per column fora main storage and one virtual file for a delta log. In addition, onevirtual file can optionally be stored per column for the main storage ofthe history part of the table, and/or one virtual file can optionally bestored per table for the delta of the history part of the table. Thepersistence layer 112 can maintain a directory that stores for eachvirtual file the start page and additional information such as the sizeand the type of the virtual file.

As stated above, virtual files can be used to store main and delta partsof columnar tables. These files can be read on the first access of thecorresponding table into memory. With some implementations, while readaccesses happen only on the in-memory representation of data, updates,appends, overwrites and truncates can also be written to the virtualfile on disk. After partitioning/moving of a virtual file from a sourcenode to a destination node, the virtual file can be read into memory onfirst access on the destination node. To support recovery from logbackup, moving a virtual file from one node to the other (if thetechniques described below are not incorporated) can either requirewriting a redo log on the destination node for all partitioned/moveddata or explicit expensive synchronization of recovery on several nodes,which is in both cases too big performance penalty.

The content of a main storage can only change when a delta mergeoperation is performed. Therefore the main virtual files can only bewritten when a merge is done. Note that this does not mean that maindata is written to disk during a merge operation: when the column store305 writes to a virtual file, the data can be written into the pagebuffer 304 of the persistence layer 112. It is the responsibility of thepersistence layer 112 to determine when the data in the virtual file isactually flushed to disk (e.g., during page replacement or at latestwhen the next savepoint is written, etc.).

A delta merge operation is unique to the column store 305 and is notsynchronized with the savepoints of the persistence layer 112. Deltamerge is primarily an optimization of in-memory structures performed onthe granularity of a single table. The savepoint, on the other hand,works on the whole database and its purpose is to persist changes todisk.

All changes executed on column store 305 data go into delta storages inthe data volumes 310. The delta storages can exist only in memory asopposed to be written to disk. However, the column store 305 can, viathe logger 344, write a persisted delta log that contains logical redolog entries for all operations executed on the delta storages. Logicallog, in this context, means that the operation and its parameters arelogged but no physical images are stored. When a delta merge operationis executed, the changes in the delta storage can be merged into themain storage and the delta log virtual file can be truncated.

Despite of the name “delta log”, the delta log virtual files are notreally logs from the persistence layer 112 point of view. For thepersistence layer 112 they are just data. The actual redo log and undoentries can be written a log volume 317 in the persistence layer 112.The virtual files used for delta logs can be configured as logged.Whenever column store 305 writes to the delta log virtual file, thepersistence layer interface 302 invokes the logger 344 and an undomanager to write redo log entries and undo information. This ensuresthat the delta log virtual files can be restored after a restart—justlike any other data. After the delta log virtual files are restored theyare ready to be processed by column store 305 to rebuild the in-memorydelta storages from the logical delta log entries.

During a delta merge operation the main files for the affected table(s)can be rewritten and the delta log file can be truncated. For all theseoperations no log is written by the persistence layer 112. This ispossible, because all operations executed on the tables were alreadylogged when the delta files were written as part of the original changeoperation. The merge operation does not change, create or delete anyinformation in the database. It is just a reorganization of the wayexisting information is stored. To prevent that logs are written for amerge operation, the virtual main files are configured as not logged anda special not logged operation is used for delta log truncation.

During restart, the persistence layer 112 can restore the main virtualfiles from the last savepoint. The delta log virtual files can berestored from the last savepoint and from the redo log. When thepersistence layer 112 has finished its part, the main storage of thecolumns can be loaded from the virtual files into column-store memory.This involves memory copy operations between data cache in the pagebuffer 304 of the persistence layer 112 and the contiguous memory areasin column store 305. The column store 305 can then execute the logicalredo entries from delta log virtual files and rebuild the in-memorydelta storages.

As mentioned above, there is metadata that allows to define for eachcolumnar table whether it is to be loaded during system startup. If atable is configured for loading on demand, the restore sequence for thattable is executed on first access.

In some situations, it can be necessary to partition data within avirtual file among two or more nodes. With reference to the diagram 400of FIG. 4, a virtual file 414 on a source node 410 within a distributeddata storage system can be partitioned by creating a new file 424 (e.g.,an empty object, etc.) on a destination node 420 containing a specialrecord 426 with (i) a link to the original virtual file 414 on thesource node 410; and (ii) partitioning criteria 428 that characterizes acorresponding partition on the original virtual file 414. This operationcan be logged by the logger 344. The original virtual file 414 can bekept unchanged on the source node. When the virtual file is read on thedestination node, the link 426 is encountered which with thepartitioning criteria 428, in turn, results in the data from thepartition of the original virtual file 410 being accessed on the sourcenode 420. New data 430 can be appended after the link record 426 on thedestination node 420, thus always all data of the file can be read. Asused in this context, nodes can refer to servers having their ownpersistency.

When performing columnar table merge operation, virtual files containingcompressed data of a corresponding table can be overwritten completelywith the new version and the virtual file for deltas will be truncated.These operations can automatically overwrite the link 426 to the sourcenode 410. During a link cleanup operation after commit, the originalfiles can be scheduled for removal. After a savepoint on the destinationnode 420 has persisted the new versions of the files to secondarystorage, old files on the source node 410 can be dropped (non-logged).At this time, files can be completely moved to the destination node 420,without unnecessary performance penalty by logging whole contents on thedestination node 420.

During recovery from log, link record 426 can be recovered as well asall new data 430 appended to the file. The portion of the old virtualfile 414 corresponding to the partition on the source node 410 can bekept because dropping of the virtual file 414 on the source node 410need not be logged. If the portion of the virtual file 414 correspondingto the partition is deleted during recovery, the deletion can scheduledropping the portion of the original file 414 corresponding to thepartition on the source node 410 after recovery ends. Otherwise, therewould be a file 424 with the link 426 to old data on the source node 420corresponding to the partition (or even a chain of links, if the virtualfile 414 has been moved several times).

The subject matter described above can be extended to enable a virtualfile to be repartitioned from n partitions to m partitions and/or tojoin n partitions to a single partition. With such variations, insteadof writing a single link on each destination node 420, several links canbe written on the destination node 420 which refer to all original npartitions where to read the source data (which can be from a pluralityof different nodes). Such an arrangement can be further optimized if anew partition of the source node 410 only a subset of the data of someof the original partitions (e.g., previous partition criteria waspartition key module 2 while the new criteria is partition key modulo 4,etc.).

Aspects of the subject matter described herein can be embodied insystems, apparatus, methods, and/or articles depending on the desiredconfiguration. In particular, various implementations of the subjectmatter described herein can be realized in digital electronic circuitry,integrated circuitry, specially designed application specific integratedcircuits (ASICs), computer hardware, firmware, software, and/orcombinations thereof. These various implementations can includeimplementation in one or more computer programs that are executableand/or interpretable on a programmable system including at least oneprogrammable processor, which can be special or general purpose, coupledto receive data and instructions from, and to transmit data andinstructions to, a storage system, at least one input device, and atleast one output device.

These computer programs, which can also be referred to programs,software, software applications, applications, components, or code,include machine instructions for a programmable processor, and can beimplemented in a high-level procedural and/or object-orientedprogramming language, and/or in assembly/machine language. As usedherein, the term “machine-readable medium” refers to any computerprogram product, apparatus and/or device, such as for example magneticdiscs, optical disks, memory, and Programmable Logic Devices (PLDs),used to provide machine instructions and/or data to a programmableprocessor, including a machine-readable medium that receives machineinstructions as a machine-readable signal. The term “machine-readablesignal” refers to any signal used to provide machine instructions and/ordata to a programmable processor. The machine-readable medium can storesuch machine instructions non-transitorily, such as for example as woulda non-transient solid state memory or a magnetic hard drive or anyequivalent storage medium. The machine-readable medium can alternativelyor additionally store such machine instructions in a transient manner,such as for example as would a processor cache or other random accessmemory associated with one or more physical processor cores.

The subject matter described herein can be implemented in a computingsystem that includes a back-end component, such as for example one ormore data servers, or that includes a middleware component, such as forexample one or more application servers, or that includes a front-endcomponent, such as for example one or more client computers having agraphical user interface or a Web browser through which a user caninteract with an implementation of the subject matter described herein,or any combination of such back-end, middleware, or front-endcomponents. A client and server are generally, but not exclusively,remote from each other and typically interact through a communicationnetwork, although the components of the system can be interconnected byany form or medium of digital data communication. Examples ofcommunication networks include, but are not limited to, a local areanetwork (“LAN”), a wide area network (“WAN”), and the Internet. Therelationship of client and server arises by virtue of computer programsrunning on the respective computers and having a client-serverrelationship to each other.

The implementations set forth in the foregoing description do notrepresent all implementations consistent with the subject matterdescribed herein. Instead, they are merely some examples consistent withaspects related to the described subject matter. Although a fewvariations have been described in detail herein, other modifications oradditions are possible. In particular, further features and/orvariations can be provided in addition to those set forth herein. Forexample, the implementations described above can be directed to variouscombinations and sub-combinations of the disclosed features and/orcombinations and sub-combinations of one or more features further tothose disclosed herein. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. The scope of the following claims may include otherimplementations or embodiments.

1. A computer program product comprising a non-transitorymachine-readable medium storing instructions that, when executed by atleast one programmable processor, cause the at least one programmableprocessor to perform operations comprising: generating, on a destinationnode of a distributed data storage system, a record within a destinationvirtual file, the record comprising (i) a link directed to a sourcevirtual file stored on a source node and (ii) partition criteriacharacterizing a partition of the source virtual file, the sourcevirtual file being mapped to a chain of linked pages stored in a pagebuffer of the distributed data storage system, the partitioning criteriabeing used by at least one of the source node and the destination nodeto identify data associated with the partition; receiving, at thedestination node, a request to access data defined by the destinationvirtual file; providing, in response to the request, data from thepartition of the source virtual file stored on the source node using thelink and the partitioning criteria; and dropping the partition of thesource virtual file after a new version of the destination virtual fileis persisted while maintaining other portions of the source virtual fileother than the partition.
 2. A computer program product as in claim 1,wherein the operations further comprise: appending data generated at thedestination node subsequent to the generation of the record to therecord, the appended data forming a delta log at the destination node.3. A computer program product as in claim 2, wherein the operationsfurther comprise: overwriting the destination virtual file and the deltalog during a columnar table merge operation with the new version of thedestination virtual file comprising data from the partition of thesource virtual file and the delta log.
 4. A computer program product asin claim 3, wherein the link in the record is overwritten during thecolumnar table merge operation.
 5. A computer program product as inclaim 3, wherein the operations further comprise: persisting the newversion of the destination virtual file to secondary data storage.
 6. Acomputer program product as in claim 5, wherein the new version of thedestination virtual file is persisted after a savepoint on thedestination node.
 7. A computer program product as in claim 5, whereinthe the new version of the destination virtual file is persisted to thesecondary data storage.
 8. A computer program product as in claim 7,wherein dropping the partition of the source virtual file comprisesceasing log operations relative to a portion of the source virtual filecorresponding to the partition.
 9. A computer program product as inclaim 8, wherein dropping the partition of the source virtual filecomprises deleting a portion of the source virtual file corresponding tothe partition.
 10. A method comprising: generating, on a destinationnode of a distributed data storage system, a record within a destinationvirtual file, the record comprising (i) a link directed to a sourcevirtual file stored on a source node and (ii) partition criteriacharacterizing a partition of the source virtual file, the sourcevirtual file being mapped to a chain of linked pages stored in a pagebuffer of the distributed data storage system, the partitioning criteriabeing used by at least one of the source node and the destination nodeto identify data associated with the partition; receiving, at thedestination node, a request to access data defined by the destinationvirtual file; providing, in response to the request, data from thepartition of the source virtual file stored on the source node using thelink and the partitioning criteria; and dropping the partition of thesource virtual file after a new version of the destination virtual fileis persisted while maintaining other portions of the source virtual fileother than the partition.
 11. A method as in claim 10, furthercomprising: appending data generated at the destination node subsequentto the generation of the record to the record, the appended data forminga delta log at the destination node.
 12. A method as in claim 11,further comprising: overwriting the destination virtual file and thedelta log during a columnar table merge operation with the new versionof the destination virtual file comprising data from the partition ofthe source virtual file and the delta log.
 13. A method as in claim 12,wherein the link in the record is overwritten during the columnar tablemerge operation.
 14. A method as in claim 12, further comprising:persisting the new version of the destination virtual file to secondarydata storage.
 15. A method as in claim 14, wherein the new version ofthe destination virtual file is persisted after a savepoint on thedestination node.
 16. A method as in claim 14, wherein the the newversion of the destination virtual file is persisted to the secondarydata storage.
 17. A method as in claim 16, wherein dropping thepartition of the source virtual file comprises ceasing log operationsrelative to a portion of the source virtual file corresponding to thepartition.
 18. A method as in claim 17, wherein dropping the partitionof the source virtual file comprises deleting the portion of the sourcevirtual file corresponding to the partition.
 19. A system comprising: atleast one data processor; and memory coupled to the at least one dataprocessor, the memory storing instructions, which when executed, causethe at least one data processor to perform operations comprising:generating, on a destination node of a distributed data storage system,a record within a destination virtual file, the record comprising (i) alink directed to a source virtual file stored on at least two sourcenodes and (ii) partition criteria characterizing a partition of thesource virtual file on one of the source nodes, the source virtual filebeing mapped to a chain of linked pages stored in a page buffer of thedistributed data storage system, the partitioning criteria being used byat least one of the source nodes and the destination node to identifydata associated with the partition; receiving, at the destination node,a request to access data defined by the destination virtual file; andproviding, in response to the request, data from the partition of thesource virtual file stored on the corresponding source node using thelink and the partitioning criteria; and dropping the partition of thesource virtual file after a new version of the destination virtual fileis persisted while maintaining other portions of the source virtual fileother than the partition.
 20. A system as in claim 19, wherein theoperations further comprise: appending data generated at the destinationnode subsequent to the generation of the record to the record, theappended data forming a delta log at the destination node; andoverwriting the destination virtual file and the delta log during acolumnar table merge operation with the new version of the destinationvirtual file comprising data from the partition of the source virtualfile and the delta log.